Variable optical retarders have been employed heretofore in stereoscopic imaging systems. For example, the article by Roese and Khalafalla, "Stereoscopic Viewing With PLZT Ceramics," Ferroelectrics, Vol. 10 (1976), pp. 47-51, describes a stereoscopic viewing system that employs lead lanthanum zirconate titanate (PLZT) ferroelectric ceramics. The stereoscopic viewing system comprises a pair of PLZT devices that form the eyepieces of glasses worn by an observer. Each such device is positioned in front of one of the observer's eyes and includes an optically cemented assembly of antireflective coated glass, a front polarizer, the PLZT ceramic wafer, and a rear polarizer. A switchable power source selectively applies 0 volts and 500 volts to each PLZT device to change its birefringence and thereby change the direction of polarization of light incident on it. Changing the polarization direction of light by the PLZT device causes either the transmission or the extinction of light carrying the image before it reaches the observer's eyes.
On page 49, the article states that the imaging system would be inoperable if the front polarizer of the PLZT device assembly were removed and a polarizing sheet were placed on the screen of the imaging source. The reason is that the resulting imaging system would have a very narrow field of view because of a degradation in optical contrast ratio as the viewing angle changes from the normal to the rear polarizer surface.
The viewing system described by Roese et al. can also present a safety problem in that it requires the use of a viewing device to which a relatively high voltage is applied and which is worn by the observer.
The article by Balasubramonian and Gunasekaran, "On the Merits of Bicircular Polarization for Stereo Color TV," IEEE Transactions on Consumer Electronics, Vol. CE-28, No. 4 (November 1982), pp. 638-650, describes a three-dimensional viewing system based on the use of left- and right-circularly polarized light for carrying images that emanate from the screens of two color cathode ray tubes. The color cathode ray tubes present together left and right perspective view images of a scene. A neutral linear polarizing filter and a fixed quarter-wave plate are positioned in front of the screen of each cathode ray tube. The optic axes of the quarter-wave plates are parallel to each other, and the transmission axes of the polarizing filters are pendicular to each other. The perspective view images propagate in left-circularly polarized light from one cathode ray tube and in right-circularly polarized light from the other cathode ray tube.
The imaging system employs a passive viewing device comprising two eyepieces, each including a quarter-wave plate and a neutral linear polarizer to remove the circular polarization and transmit the left view image to the left eye and the right view image to the right eye. The optic axes of the quarter-wave plates are parallel to each other and parallel to the optic axes of the quarter-wave plates positioned in front of the cathode ray tube screens. The neutral linear polarizers have orthogonally aligned absorption axes. The article states that the use of left- and right-circular polarization provides three-dimensional perception that is unrestricted by the observer's head movements.
The system of Balasubramonian et al. suffers from the disadvantage of requiring two cathode ray tubes to develop a stereoscopic image.
U.S. Pat. No. 4,281,341 of Byatt describes a receiver in a stereoscopic television system that employs a twisted nematic liquid crystal cell to change the polarization direction of light carrying perspective view images emanating in field sequential format from a cathode ray tube. The twisted nematic device is positioned downstream of a linear polarizing filter which is positioned in front of the screen of the cathode ray tube and which has its transmission axis oriented in the vertical direction. An observer wears passive glasses with two eyepieces, each comprising a neutral polarizing filter. The polarization axes of the neutral polarizing filters are orthogonally aligned, with one of them aligned parallel to the polarizing filter positioned in front of the cathode ray tube screen.
The Byatt system suffers from the disadvantage of requiring the application of a relatively high voltage to the twisted nematic cell to change rapidly the polarization direction of the light carrying the perspective view images. Use of such a device at slow switching speeds causes image flicker, and use of a slow device at relatively high switching speeds develops a low contrast image.